1. Field of the Invention
The present invention relates to a burner useful for operating a heat generator including a first upstream swirl generator capable of swirling a combustion air stream, means for injecting at least one fuel into the combustion air stream from the upstream swirl generator, an exit ring located at the downstream end of the burner at the edge to the combustion chamber where the fuel is burnt.
2. Brief Description of the Related Art
Premixed burners are characterized by a particularly low emission of NOx if operated under lean conditions. Correspondingly, these burners are operated under lean conditions at standard load. If the load is reduced, these burners have the tendency to become unstable when the supply with fuel is reduced.
A premixed burner is e.g. proposed in EP 0 321 809 B1, comprising several conical wall portions which are shifted with respect to each other leaving entrance slots through which the combustion air is entering the interior of the burner. Liquid and gaseous fuels can be burnt in such a premixed burner, preferentially liquid fuel is injected by means of a central fuel nozzle located on the axis of the burner, while gaseous fuels can be added to the stream of combustion air at the entrance slots between the conical wall portions.
An alternative premixed burner, which is described in e.g. EP 0 704 657 A2, or in EP 0 780 629 A2, additionally comprises a mixing tube located downstream of a burner as described in EP 0 321 809 B1, wherein at the entrance of the mixing tube there are transfer ducts for a controlled entrance of the swirling combustion air into the mixing tube.
To allow reduction of the fuel supply without the above-mentioned problems, a so-called pilot mode is made possible for such burners by providing particular pilot nozzles at the central fuel nozzle or by providing particularly long central fuel nozzles. Alternatively, as described in EP 0 797 051 A2, pilot mode can be made possible by providing, next to the burner, on the backside wall of the combustion chamber and distanced from the exit of the burner, separate mixing elements for fuel and combustion air which can be used for pilot operation of the burner. Another possibility for pilot operation is described in EP 0 994 300 A1, where a burner according to EP 0 704 657 A2 or EP 0 780 629 A2 is provided with an exit ring comprising swirl generators, and where pilot gas is injected into the combustion chamber into the swirl formed by these swirl generators. Yet another alternative for pilot operation is described in EP 0 931 980 A1, where pilot gas is injected next to the exit ring into the combustion chamber after mixing it with combustion air. Additionally, means for igniting the pilot gas are described in this document.